Produced by Amy E. Zelmer





YEAST

By Thomas H. Huxley



I HAVE selected to-night the particular subject of Yeast for two
reasons--or, rather, I should say for three. In the first place, because
it is one of the simplest and the most familiar objects with which we
are acquainted. In the second place, because the facts and phenomena
which I have to describe are so simple that it is possible to put them
before you without the help of any of those pictures or diagrams which
are needed when matters are more complicated, and which, if I had to
refer to them here, would involve the necessity of my turning away from
you now and then, and thereby increasing very largely my difficulty
(already sufficiently great) in making myself heard. And thirdly, I have
chosen this subject because I know of no familiar substance forming part
of our every-day knowledge and experience, the examination of which,
with a little care, tends to open up such very considerable issues as
does this substance--yeast.

In the first place, I should like to call your attention to a fact with
which the whole of you are, to begin with, perfectly acquainted, I mean
the fact that any liquid containing sugar, any liquid which is formed by
pressing out the succulent parts of the fruits of plants, or a mixture
of honey and water, if left to itself for a short time, begins to
undergo a peculiar change. No matter how clear it might be at starting,
yet after a few hours, or at most a few days, if the temperature is
high, this liquid begins to be turbid, and by-and-by bubbles make their
appearance in it, and a sort of dirty-looking yellowish foam or scum
collects at the surface; while at the same time, by degrees, a similar
kind of matter, which we call the "lees," sinks to the bottom.

The quantity of this dirty-looking stuff, that we call the scum and the
lees, goes on increasing until it reaches a certain amount, and then
it stops; and by the time it stops, you find the liquid in which this
matter has been formed has become altered in its quality. To begin with
it was a mere sweetish substance, having the flavour of whatever might
be the plant from which it was expressed, or having merely the taste and
the absence of smell of a solution of sugar; but by the time that this
change that I have been briefly describing to you is accomplished the
liquid has become completely altered, it has acquired a peculiar smell,
and, what is still more remarkable, it has gained the property of
intoxicating the person who drinks it. Nothing can be more innocent than
a solution of sugar; nothing can be less innocent, if taken in excess,
as you all know, than those fermented matters which are produced
from sugar. Well, again, if you notice that bubbling, or, as it were,
seething of the liquid, which has accompanied the whole of this process,
you will find that it is produced by the evolution of little bubbles of
air-like substance out of the liquid; and I dare say you all know this
air-like substance is not like common air; it is not a substance which
a man can breathe with impunity. You often hear of accidents which take
place in brewers' vats when men go in carelessly, and get suffocated
there without knowing that there was anything evil awaiting them. And if
you tried the experiment with this liquid I am telling of while it
was fermenting, you would find that any small animal let down into the
vessel would be similarly stifled; and you would discover that a light
lowered down into it would go out. Well, then, lastly, if after this
liquid has been thus altered you expose it to that process which is
called distillation; that is to say, if you put it into a still, and
collect the matters which are sent over, you obtain, when you first heat
it, a clear transparent liquid, which, however, is something totally
different from water; it is much lighter; it has a strong smell, and it
has an acrid taste; and it possesses the same intoxicating power as the
original liquid, but in a much more intense degree. If you put a light
to it, it burns with a bright flame, and it is that substance which we
know as spirits of wine.

Now these facts which I have just put before you--all but the last--have
been known from extremely remote antiquity. It is, I hope one of the
best evidences of the antiquity of the human race, that among the
earliest records of all kinds of men, you find a time recorded when they
got drunk. We may hope that that must have been a very late period in
their history. Not only have we the record of what happened to Noah, but
if we turn to the traditions of a different people, those forefathers
of ours who lived in the high lands of Northern India, we find that they
were not less addicted to intoxicating liquids; and I have no doubt
that the knowledge of this process extends far beyond the limits of
historically recorded time. And it is a very curious thing to observe
that all the names we have of this process, and all that belongs to
it, are names that have their roots not in our present language, but in
those older languages which go back to the times at which this country
was peopled. That word "fermentation" for example, which is the title
we apply to the whole process, is a Latin term; and a term which is
evidently based upon the fact of the effervescence of the liquid. Then
the French, who are very fond of calling themselves a Latin race, have a
particular word for ferment, which is 'levure'. And, in the same way, we
have the word "leaven," those two words having reference to the heaving
up, or to the raising of the substance which is fermented. Now those are
words which we get from what I may call the Latin side of our parentage;
but if we turn to the Saxon side, there are a number of names connected
with this process of fermentation. For example, the Germans call
fermentation--and the old Germans did so--"gahren;" and they call
anything which is used as a ferment by such names, such as "gheist" and
"geest," and finally in low German, "yest"; and that word you know is
the word our Saxon forefathers used, and is almost the same as the word
which is commonly employed in this country to denote the common ferment
of which I have been speaking. So they have another name, the word
"hefe," which is derived from their verb "heben," which signifies to
raise up; and they have yet a third name, which is also one common in
this country (I do not know whether it is common in Lancashire, but it
is certainly very common in the Midland countries), the word "barm,"
which is derived from a root which signifies to raise or to bear up.
Barm is a something borne up; and thus there is much more real relation
than is commonly supposed by those who make puns, between the beer which
a man takes down his throat and the bier upon which that process, if
carried to excess, generally lands him, for they are both derived
from the root signifying bearing up; the one thing is borne upon men's
shoulders, and the other is the fermented liquid which was borne up by
the fermentation taking place in itself.

Again, I spoke of the produce of fermentation as "spirit of wine." Now
what a very curious phrase that is, if you come to think of it. The old
alchemists talked of the finest essence of anything as if it had the
same sort of relation to the thing itself as a man's spirit is supposed
to have to his body; and so they spoke of this fine essence of the
fermented liquid as being the spirit of the liquid. Thus came about
that extraordinary ambiguity of language, in virtue of which you apply
precisely the same substantive name to the soul of man and to a glass
of gin! And then there is still yet one other most curious piece of
nomenclature connected with this matter, and that is the word "alcohol"
itself, which is now so familiar to everybody. Alcohol originally meant
a very fine powder. The women of the Arabs and other Eastern people are
in the habit of tinging their eyelashes with a very fine black powder
which is made of antimony, and they call that "kohol;" and the "al" is
simply the article put in front of it, so as to say "the kohol." And
up to the 17th century in this country the word alcohol was employed to
signify any very fine powder; you find it in Robert Boyle's works that
he uses "alcohol" for a very fine subtle powder. But then this name of
anything very fine and very subtle came to be specially connected with
the fine and subtle spirit obtained from the fermentation of sugar; and
I believe that the first person who fairly fixed it as the proper name
of what we now commonly call spirits of wine, was the great French
chemist Lavoisier, so comparatively recent is the use of the word
alcohol in this specialised sense.

So much by way of general introduction to the subject on which I have to
speak to-night. What I have hitherto stated is simply what we may call
common knowledge, which everybody may acquaint himself with. And
you know that what we call scientific knowledge is not any kind
of conjuration, as people sometimes suppose, but it is simply the
application of the same principles of common sense that we apply to
common knowledge, carried out, if I may so speak, to knowledge which is
uncommon. And all that we know now of this substance, yeast, and all the
very strange issues to which that knowledge has led us, have simply come
out of the inveterate habit, and a very fortunate habit for the human
race it is, which scientific men have of not being content until they
have routed out all the different chains and connections of apparently
simple phenomena, until they have taken them to pieces and understood
the conditions upon which they depend. I will try to point out to you
now what has happened in consequence of endeavouring to apply this
process of "analysis," as we call it, this teazing out of an apparently
simple fact into all the little facts of which it is made up, to the
ascertained facts relating to the barm or the yeast; secondly, what has
come of the attempt to ascertain distinctly what is the nature of the
products which are produced by fermentation; then what has come of the
attempt to understand the relation between the yeast and the products;
and lastly, what very curious side issues if I may so call them--have
branched out in the course of this inquiry, which has now occupied
somewhere about two centuries.

The first thing was to make out precisely and clearly what was the
nature of this substance, this apparently mere scum and mud that we
call yeast. And that was first commenced seriously by a wonderful old
Dutchman of the name of Leeuwenhoek, who lived some two hundred years
ago, and who was the first person to invent thoroughly trustworthy
microscopes of high powers. Now, Leeuwenhoek went to work upon this
yeast mud, and by applying to it high powers of the microscope, he
discovered that it was no mere mud such as you might at first suppose,
but that it was a substance made up of an enormous multitude of minute
grains, each of which had just as definite a form as if it were a grain
of corn, although it was vastly smaller, the largest of these not being
more than the two-thousandth of an inch in diameter; while, as you
know, a grain of corn is a large thing, and the very smallest of
these particles were not more than the seven-thousandth of an inch in
diameter. Leeuwenhoek saw that this muddy stuff was in reality a liquid,
in which there were floating this immense number of definitely shaped
particles, all aggregated in heaps and lumps and some of them separate.
That discovery remained, so to speak, dormant for fully a century, and
then the question was taken up by a French discoverer, who, paying
great attention and having the advantage of better instruments than
Leeuwenhoek had, watched these things and made the astounding discovery
that they were bodies which were constantly being reproduced and
growing; than when one of these rounded bodies was once formed and had
grown to its full size, it immediately began to give off a little bud
from one side, and then that bud grew out until it had attained the
full size of the first, and that, in this way, the yeast particle was
undergoing a process of multiplication by budding, just as effectual and
just as complete as the process of multiplication of a plant by
budding; and thus this Frenchman, Cagniard de la Tour, arrived at
the conclusion--very creditable to his sagacity, and which has been
confirmed by every observation and reasoning since--that this apparently
muddy refuse was neither more nor less than a mass of plants, of minute
living plants, growing and multiplying in the sugary fluid in which the
yeast is formed. And from that time forth we have known this substance
which forms the scum and the lees as the yeast plant; and it has
received a scientific name--which I may use without thinking of it,
and which I will therefore give you--namely, "Torula." Well, this was a
capital discovery. The next thing to do was to make out how this torula
was related to the other plants. I won't weary you with the whole course
of investigation, but I may sum up its results, and they are these--that
the torula is a particular kind of a fungus, a particular state
rather, of a fungus or mould. There are many moulds which under certain
conditions give rise to this torula condition, to a substance which is
not distinguishable from yeast, and which has the same properties as
yeast--that is to say, which is able to decompose sugar in the curious
way that we shall consider by-and-by. So that the yeast plant is a plant
belonging to a group of the Fungi, multiplying and growing and living in
this very remarkable manner in the sugary fluid which is, so to speak,
the nidus or home of the yeast.

That, in a few words, is, as far as investigation--by the help of one's
eye and by the help of the microscope--has taken us. But now there is an
observer whose methods of observation are more refined than those of men
who use their eye, even though it be aided by the microscope; a man who
sees indirectly further than we can see directly--that is, the chemist;
and the chemist took up this question, and his discovery was not less
remarkable than that of the microscopist. The chemist discovered that
the yeast plant being composed of a sort of bag, like a bladder, inside
which is a peculiar soft, semifluid material--the chemist found that
this outer bladder has the same composition as the substance of wood,
that material which is called "cellulose," and which consists of the
elements carbon and hydrogen and oxygen, without any nitrogen. But then
he also found (the first person to discover it was an Italian chemist,
named Fabroni, in the end of the last century) that this inner matter
which was contained in the bag, which constitutes the yeast plant, was a
substance containing the elements carbon and hydrogen and oxygen and
nitrogen; that it was what Fabroni called a vegeto-animal substance, and
that it had the peculiarities of what are commonly called "animal
products."

This again was an exceedingly remarkable discovery. It lay neglected
for a time, until it was subsequently taken up by the great chemists of
modern times, and they, with their delicate methods of analysis, have
finally decided that, in all essential respects, the substance which
forms the chief part of the contents of the yeast plant is identical
with the material which forms the chief part of our own muscles, which
forms the chief part of our own blood, which forms the chief part of
the white of the egg; that, in fact, although this little organism is
a plant, and nothing but a plant, yet that its active living contents
contain a substance which is called "protein," which is of the same
nature as the substance which forms the foundation of every animal
organism whatever.

Now we come next to the question of the analysis of the products, of
that which is produced during the process of fermentation. So far back
as the beginning of the 16th century, in the times of transition between
the old alchemy and the modern chemistry, there was a remarkable man,
Von Helmont, a Dutchman, who saw the difference between the air which
comes out of a vat where something is fermenting and common air. He was
the man who invented the term "gas," and he called this kind of gas "gas
silvestre"--so to speak gas that is wild, and lives in out of the way
places--having in his mind the identity of this particular kind of air
with that which is found in some caves and cellars. Then, the gradual
process of investigation going on, it was discovered that this
substance, then called "fixed air," was a poisonous gas, and it was
finally identified with that kind of gas which is obtained by burning
charcoal in the air, which is called "carbonic acid." Then the
substance alcohol was subjected to examination, and it was found to be
a combination of carbon, and hydrogen, and oxygen. Then the sugar which
was contained in the fermenting liquid was examined and that was found
to contain the three elements carbon, hydrogen, and oxygen. So that
it was clear there were in sugar the fundamental elements which are
contained in the carbonic acid, and in the alcohol. And then came that
great chemist Lavoisier, and he examined into the subject carefully,
and possessed with that brilliant thought of his which happens to be
propounded exactly apropos to this matter of fermentation--that no
matter is ever lost, but that matter only changes its form and changes
its combinations--he endeavoured to make out what became of the sugar
which was subjected to fermentation. He thought he discovered that the
whole weight of the sugar was represented by the carbonic acid produced;
that in other words, supposing this tumbler to represent the sugar, that
the action of fermentation was as it were the splitting of it, the one
half going away in the shape of carbonic acid, and the other half going
away in the shape of alcohol. Subsequent inquiry, careful research with
the refinements of modern chemistry, have been applied to this problem,
and they have shown that Lavoisier was not quite correct; that what he
says is quite true for about 95 per cent. of the sugar, but that the
other 5 per cent., or nearly so, is converted into two other things;
one of them, matter which is called succinic acid, and the other
matter which is called glycerine, which you all know now as one of the
commonest of household matters. It may be that we have not got to the
end of this refined analysis yet, but at any rate, I suppose I may
say--and I speak with some little hesitation for fear my friend
Professor Roscoe here may pick me up for trespassing upon his
province--but I believe I may say that now we can account for 99 per
cent. at least of the sugar, and that 99 per cent. is split up into
these four things, carbonic acid, alcohol, succinic acid, and glycerine.
So that it may be that none of the sugar whatever disappears, and
that only its parts, so to speak, are re-arranged, and if any of it
disappears, certainly it is a very small portion.

Now these are the facts of the case. There is the fact of the growth of
the yeast plant; and there is the fact of the splitting up of the sugar.
What relation have these two facts to one another?

For a very long time that was a great matter of dispute. The early
French observers, to do them justice, discerned the real state of the
case, namely, that there was a very close connection between the actual
life of the yeast plant and this operation of the splitting up of the
sugar; and that one was in some way or other connected with the other.
All investigation subsequently has confirmed this original idea. It has
been shown that if you take any measures by which other plants of like
kind to the torula would be killed, and by which the yeast plant is
killed, then the yeast loses its efficiency. But a capital experiment
upon this subject was made by a very distinguished man, Helmholz, who
performed an experiment of this kind. He had two vessels--one of them we
will suppose full of yeast, but over the bottom of it, as this might be,
was tied a thin film of bladder; consequently, through that thin film of
bladder all the liquid parts of the yeast would go, but the solid parts
would be stopped behind; the torula would be stopped, the liquid parts
of the yeast would go. And then he took another vessel containing a
fermentable solution of sugar, and he put one inside the other; and in
this way you see the fluid parts of the yeast were able to pass through
with the utmost ease into the sugar, but the solid parts could not get
through at all. And he judged thus: if the fluid parts are those which
excite fermentation, then, inasmuch as these are stopped, the sugar will
not ferment; and the sugar did not ferment, showing quite clearly,
that an immediate contact with the solid, living torula was absolutely
necessary to excite this process of splitting up of the sugar. This
experiment was quite conclusive as to this particular point, and has had
very great fruits in other directions.

Well, then, the yeast plant being essential to the production of
fermentation, where does the yeast plant come from? Here, again, was
another great problem opened up, for, as I said at starting, you have,
under ordinary circumstances in warm weather, merely to expose some
fluid containing a solution of sugar, or any form of syrup or vegetable
juice to the air, in order, after a comparatively short time, to see all
these phenomena of fermentation. Of course the first obvious suggestion
is, that the torula has been generated within the fluid. In fact, it
seems at first quite absurd to entertain any other conviction; but that
belief would most assuredly be an erroneous one.

Towards the beginning of this century, in the vigorous times of the old
French wars, there was a Monsieur Appert, who had his attention directed
to the preservation of things that ordinarily perish, such as meats and
vegetables, and in fact he laid the foundation of our modern method of
preserving meats; and he found that if he boiled any of these substances
and then tied them so as to exclude the air, that they would be
preserved for any time. He tried these experiments, particularly with
the must of wine and with the wort of beer; and he found that if the
wort of beer had been carefully boiled and was stopped in such a way
that the air could not get at it, it would never ferment. What was the
reason of this? That, again, became the subject of a long string of
experiments, with this ultimate result, that if you take precautions to
prevent any solid matters from getting into the must of wine or the wort
of beer, under these circumstances--that is to say, if the fluid has
been boiled and placed in a bottle, and if you stuff the neck of the
bottle full of cotton wool, which allows the air to go through and stops
anything of a solid character however fine, then you may let it be for
ten years and it will not ferment. But if you take that plug out and
give the air free access, then, sooner or later fermentation will set
up. And there is no doubt whatever that fermentation is excited only by
the presence of some torula or other, and that that torula proceeds in
our present experience, from pre-existing torulae. These little bodies
are excessively light. You can easily imagine what must be the weight of
little particles, but slightly heavier than water, and not more than the
two-thousandth or perhaps seven-thousandth of an inch in diameter. They
are capable of floating about and dancing like motes in the sunbeam;
they are carried about by all sorts of currents of air; the great
majority of them perish; but one or two, which may chance to enter into
a sugary solution, immediately enter into active life, find there the
conditions of their nourishment, increase and multiply, and may give
rise to any quantity whatever of this substance yeast. And, whatever
may be true or not be true about this "spontaneous generation," as it
is called in regard to all other kinds of living things, it is perfectly
certain, as regards yeast, that it always owes its origin to this
process of transportation or inoculation, if you like so to call it,
from some other living yeast organism; and so far as yeast is concerned,
the doctrine of spontaneous generation is absolutely out of court.
And not only so, but the yeast must be alive in order to exert these
peculiar properties. If it be crushed, if it be heated so far that its
life is destroyed, that peculiar power of fermentation is not excited.
Thus we have come to this conclusion, as the result of our inquiry, that
the fermentation of sugar, the splitting of the sugar into alcohol and
carbonic acid, glycerine, and succinic acid, is the result of nothing
but the vital activity of this little fungus, the torula.

And now comes the further exceedingly difficult inquiry--how is it
that this plant, the torula, produces this singular operation of the
splitting up of the sugar? Fabroni, to whom I referred some time ago,
imagined that the effervescence of fermentation was produced in just the
same way as the effervescence of a sedlitz powder, that the yeast was a
kind of acid, and that the sugar was a combination of carbonic acid and
some base to form the alcohol, and that the yeast combined with
this substance, and set free the carbonic acid; just as when you add
carbonate of soda to acid you turn out the carbonic acid. But of course
the discovery of Lavoisier that the carbonic acid and the alcohol taken
together are very nearly equal in weight to the sugar, completely upset
this hypothesis. Another view was therefore taken by the French chemist,
Thenard, and it is still held by a very eminent chemist, M. Pasteur, and
their view is this, that the yeast, so to speak, eats a little of the
sugar, turns a little of it to its own purposes, and by so doing gives
such a shape to the sugar that the rest of it breaks up into carbonic
acid and alcohol.

Well, then, there is a third hypothesis, which is maintained by another
very distinguished chemist, Liebig, which denies either of the other
two, and which declares that the particles of the sugar are, as it were,
shaken asunder by the forces at work in the yeast plant. Now I am not
going to take you into these refinements of chemical theory, I cannot
for a moment pretend to do so, but I may put the case before you by an
analogy. Suppose you compare the sugar to a card house, and suppose you
compare the yeast to a child coming near the card house, then Fabroni's
hypothesis was that the child took half the cards away; Thenard's and
Pasteur's hypothesis is that the child pulls out the bottom card and
thus makes it tumble to pieces; and Liebig's hypothesis is that the
child comes by and shakes the table and tumbles the house down. I
appeal to my friend here (Professor Roscoe) whether that is not a fair
statement of the case.

Having thus, as far as I can, discussed the general state of the
question, it remains only that I should speak of some of those
collateral results which have come in a very remarkable way out of the
investigation of yeast. I told you that it was very early observed that
the yeast plant consisted of a bag made up of the same material as that
which composes wood, and of an interior semifluid mass which contains
a substance, identical in its composition, in a broad sense, with
that which constitutes the flesh of animals. Subsequently, after
the structure of the yeast plant had been carefully observed, it was
discovered that all plants, high and low, are made up of separate
bags or "cells," as they are called; these bags or cells having the
composition of the pure matter of wood; having the same composition,
broadly speaking, as the sac of the yeast plant, and having in their
interior a more or less fluid substance containing a matter of the same
nature as the protein substance of the yeast plant. And therefore this
remarkable result came out--that however much a plant may differ from
an animal, yet that the essential constituent of the contents of these
various cells or sacs of which the plant is made up, the nitrogenous
protein matter, is the same in the animal as in the plant. And not only
was this gradually discovered, but it was found that these semifluid
contents of the plant cell had, in many cases, a remarkable power of
contractility quite like that of the substance of animals. And about
24 or 25 years ago, namely, about the year 1846, to the best of my
recollection, a very eminent German botanist, Hugo Von Mohl, conferred
upon this substance which is found in the interior of the plant cell,
and which is identical with the matter found in the inside of the yeast
cell, and which again contains an animal substance similar to that of
which we ourselves are made up--he conferred upon this that title of
"protoplasm," which has brought other people a great deal of trouble
since! I beg particularly to say that, because I find many people
suppose that I was the inventor of that term, whereas it has been in
existence for at least twenty-five years. And then other observers,
taking the question up, came to this astonishing conclusion (working
from this basis of the yeast), that the differences between animals and
plants are not so much in the fundamental substances which compose them,
not in the protoplasm, but in the manner in which the cells of which
their bodies are built up have become modified. There is a sense in
which it is true--and the analogy was pointed out very many years ago by
some French botanists and chemists--there is a sense in which it is
true that every plant is substantially an enormous aggregation of
bodies similar to yeast cells, each having to a certain extent its own
independent life. And there is a sense in which it is also perfectly
true--although it would be impossible for me to give the statement
to you with proper qualifications and limitations on an occasion like
this--but there is also a sense in which it is true that every animal
body is made up of an aggregation of minute particles of protoplasm,
comparable each of them to the individual separate yeast plant. And
those who are acquainted with the history of the wonderful revolution
which has been worked in our whole conception of these matters in the
last thirty years, will bear me out in saying that the first germ of
them, to a very great extent, was made to grow and fructify by the study
of the yeast plant, which presents us with living matter in almost its
simplest condition.

Then there is yet one last and most important bearing of this yeast
question. There is one direction probably in which the effects of the
careful study of the nature of fermentation will yield results more
practically valuable to mankind than any other. Let me recall to your
minds the fact which I stated at the beginning of this lecture. Suppose
that I had here a solution of pure sugar with a little mineral matter
in it; and suppose it were possible for me to take upon the point of a
needle one single, solitary yeast cell, measuring no more perhaps than
the three-thousandth of an inch in diameter--not bigger than one of
those little coloured specks of matter in my own blood at this moment,
the weight of which it would be difficult to express in the fraction
of a grain--and put it into this solution. From that single one, if the
solution were kept at a fair temperature in a warm summer's day, there
would be generated, in the course of a week, enough torulae to form
a scum at the top and to form lees at the bottom, and to change the
perfectly tasteless and entirely harmless fluid, syrup, into a solution
impregnated with the poisonous gas carbonic acid, impregnated with the
poisonous substance alcohol; and that, in virtue of the changes worked
upon the sugar by the vital activity of these infinitesimally small
plants. Now you see that this is a case of infection. And from the time
that the phenomenon of fermentation were first carefully studied, it
has constantly been suggested to the minds of thoughtful physicians that
there was a something astoundingly similar between this phenomena of
the propagation of fermentation by infection and contagion, and the
phenomena of the propagation of diseases by infection and contagion.
Out of this suggestion has grown that remarkable theory of many diseases
which has been called the "germ theory of disease," the idea, in fact,
that we owe a great many diseases to particles having a certain life of
their own, and which are capable of being transmitted from one living
being to another, exactly as the yeast plant is capable of being
transmitted from one tumbler of saccharine substance to another. And
that is a perfectly tenable hypothesis, one which in the present state
of medicine ought to be absolutely exhausted and shown not to be true,
until we take to others which have less analogy in their favour. And
there are some diseases most assuredly in which it turns out to be
perfectly correct. There are some forms of what are called malignant
carbuncle which have been shown to be actually effected by a sort of
fermentation, if I may use the phrase, by a sort of disturbance and
destruction of the fluids of the animal body, set up by minute organisms
which are the cause of this destruction and of this disturbance; and
only recently the study of the phenomena which accompany vaccination
has thrown an immense light in this direction, tending to show by
experiments of the same general character as that to which I referred as
performed by Helmholz, that there is a most astonishing analogy between
the contagion of that healing disease and the contagion of destructive
diseases. For it has been made out quite clearly, by investigations
carried on in France and in this country, that the only part of the
vaccine matter which is contagious, which is capable of carrying on its
influence in the organism of the child who is vaccinated, is the solid
particles and not the fluid. By experiments of the most ingenious kind,
the solid parts have been separated from the fluid parts, and it has
then been discovered that you may vaccinate a child as much as you like
with the fluid parts, but no effect takes place, though an excessively
small portion of the solid particles, the most minute that can be
separated, is amply sufficient to give rise to all the phenomena of
the cow pock, by a process which we can compare to nothing but the
transmission of fermentation from one vessel into another, by the
transport to the one of the torula particles which exist in the other.
And it has been shown to be true of some of the most destructive
diseases which infect animals, such diseases as the sheep pox, such
diseases as that most terrible and destructive disorder of horses,
glanders, that in these, also, the active power is the living solid
particle, and that the inert part is the fluid. However, do not suppose
that I am pushing the analogy too far. I do not mean to say that the
active, solid parts in these diseased matters are of the same nature as
living yeast plants; but, so far as it goes, there is a most surprising
analogy between the two; and the value of the analogy is this, that by
following it out we may some time or other come to understand how these
diseases are propagated, just as we understand, now, about fermentation;
and that, in this way, some of the greatest scourges which afflict the
human race may be, if not prevented, at least largely alleviated.

This is the conclusion of the statements which I wished to put before
you. You see we have not been able to have any accessories. If you will
come in such numbers to hear a lecture of this kind, all I can say is,
that diagrams cannot be made big enough for you, and that it is not
possible to show any experiments illustrative of a lecture on such a
subject as I have to deal with. Of course my friends the chemists and
physicists are very much better off, because they can not only show you
experiments, but you can smell them and hear them! But in my case such
aids are not attainable, and therefore I have taken a simple subject and
have dealt with it in such a way that I hope you all understand it,
at least so far as I have been able to put it before you in words; and
having once apprehended such of the ideas and simple facts of the case
as it was possible to put before you, you can see for yourselves the
great and wonderful issues of such an apparently homely subject.